U.S. patent number 7,654,157 [Application Number 11/948,317] was granted by the patent office on 2010-02-02 for airflow sensor with pitot tube for pressure drop reduction.
This patent grant is currently assigned to Honeywell International Inc.. Invention is credited to Jamie W. Speldrich.
United States Patent |
7,654,157 |
Speldrich |
February 2, 2010 |
Airflow sensor with pitot tube for pressure drop reduction
Abstract
An airflow sensor apparatus for measuring flow rate includes a
pitot tube with a bypass channel wherein the pitot tube extends
halfway into a flow channel in order to reduce a pressure drop. One
or more upstream taps can be spaced along the pitot tube facing
into a direction of a flow stream which directs the flow to the
bypass channel. At least one or more downstream taps can be located
to face perpendicular to the direction of flow, such that the fluid
after passing over a flow sensor passes through the downstream
tap(s). The upstream tap senses stagnation pressure and the down
stream tap senses static pressure which is exerted in all
directions in the flow channel in order to determine a velocity
pressure based on a difference between pressures.
Inventors: |
Speldrich; Jamie W. (Freeport,
IL) |
Assignee: |
Honeywell International Inc.
(Morristown, NJ)
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Family
ID: |
40674413 |
Appl.
No.: |
11/948,317 |
Filed: |
November 30, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090139348 A1 |
Jun 4, 2009 |
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Current U.S.
Class: |
73/861.65 |
Current CPC
Class: |
G01F
5/00 (20130101); G01F 1/6842 (20130101); G01F
1/46 (20130101) |
Current International
Class: |
G01F
1/46 (20060101) |
Field of
Search: |
;73/861.65-861.66 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0255056 |
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Dec 1991 |
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EP |
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9221940 |
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Dec 1992 |
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WO |
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Primary Examiner: Patel; Harshad
Claims
What is claimed is:
1. A flow sensor apparatus for measuring a property related to
fluid flow through a flow channel, wherein the cross-sectional size
of the flow channel is defined by one or more flow channel inner
dimensions, comprising: a housing having at least a portion that
extends into the flow channel, but substantially less than all the
way across an inner dimension of the flow channel; the housing
defining a pitot tube with a bypass channel; the pitot tube having
at least two upstream taps spaced apart along said pitot tube and
oriented into a direction of fluid flow through said flow channel,
the at least two upstream taps collectively functioning as an
upstream port for said bypass channel; the pitot tube further
having at least two downstream taps spaced apart along said pitot
tube and facing perpendicular or substantially perpendicular to
said direction of said fluid flow, the at least two downstream taps
collectively functioning as a downstream port for said bypass
channel; and the bypass channel is configured to direct said fluid
from said upstream port, to a sensor that is configured to sense a
property related to fluid flow through the flow channel, and
finally to the at least two downstream taps.
2. The flow sensor apparatus of claim 1, wherein said housing
further comprises an inlet end and an outlet end that are
configured for coupling the flow sensor apparatus to a flow
system.
3. The flow sensor apparatus of claim 1, wherein said sensor
comprises a MEMS flow sensor with an upstream sensing element, a
downstream sensing element and a heating element.
4. The flow sensor apparatus of claim 1 wherein said housing
extends about halfway across the inner dimension of the flow
channel.
5. The flow sensor apparatus of claim 1 wherein said Pitot tube has
a tapered leading edge for reducing turbulence in the flow
channel.
6. A flow sensor apparatus, comprising: a housing including a wall
defining a pitot tube including a bypass channel through which
fluid is enabled to flow, said pitot tube defined by said housing
and extending into but substantially less than all the way across a
width dimension of a flow channel; at least two upstream taps
spaced apart along said pitot tube facing into the direction of a
fluid flowing through the flow channel, the at least two upstream
taps collectively functioning as an upstream port for said bypass
channel; at least two downstream taps spaced apart along said pitot
tube and facing perpendicular or substantially perpendicular to
said direction of said fluid flow, the at least two downstream taps
collectively functioning as a downstream port for said bypass
channel; and the bypass channel is configured to direct said fluid
from said upstream port, to a sensor that is configured to sense a
property related to fluid flow through the flow channel, and
finally to the two or more downstream taps.
7. The flow sensor apparatus of claim 6, wherein said housing
comprises an inlet end and an outlet end that are configured for
coupling the flow sensor apparatus to a flow system.
8. The flow sensor apparatus of claim 6 wherein said sensor
comprises a MEMS flow sensor with an upstream sensing element, a
downstream sensing element and a heating element.
9. The flow sensor apparatus of claim 6 wherein said housing
extends transversely about halfway across the width dimension of
said flow channel.
10. The flow sensor apparatus of claim 6 wherein said Pitot tube
has a tapered leading edge for reducing turbulence in the flow
channel.
11. A flow sensor apparatus, comprising: a pitot tube having a
bypass channel, said pitot tube extending transversely into but
substantially less than all the way across a flow channel with
respect to the direction of flow of a fluid within said flow
channel; at least two upstream taps spaced apart along said pitot
tube facing into a direction of a fluid flowing through said flow
channel, the at least two upstream taps collectively functioning as
an upstream port for said bypass channel; a flow sensor configured
to sense the flow of said fluid provided by said upstream port; and
at least one downstream tap spaced apart along said pitot tube and
facing perpendicular or substantially perpendicular to said
direction of said fluid in order to determine a velocity pressure
based on a difference between a pressure identified by said at
least two upstream taps and a pressure identified by said at least
one downstream tap.
12. The flow sensor apparatus of claim 11 further comprising: a
housing that defines said pitot tube with said bypass channel.
13. The flow sensor apparatus of claim 12, wherein said housing
further comprises an inlet end and an outlet end that are
configured for coupling the flow sensor apparatus to a flow
system.
14. The flow sensor apparatus of claim 13, wherein said sensor
comprises a MEMS flow sensor with an upstream sensing element, a
downstream sensing element and a heating element.
15. The flow sensor apparatus of claim 12 wherein said housing
extends about halfway across the flow channel.
16. The flow sensor apparatus of claim 15 wherein said housing has
a tapered leading edge for reducing turbulence and pressure drop in
the flow channel.
17. The flow sensor apparatus of claim 11, wherein said sensor
comprises a MEMS flow sensor with an upstream sensing element, a
downstream sensing element and a heating element.
18. The flow sensor apparatus of claim 11 wherein said pitot tube
extends about halfway across the flow channel.
19. The flow sensor apparatus of claim 11 wherein said Pitot tube
has a tapered leading edge for reducing turbulence and pressure
drop in the flow channel.
Description
TECHNICAL FIELD
Embodiments are generally related to sensor methods and systems.
Embodiments are also related to airflow sensors for medical
ventilator applications. Embodiments are additionally related to
airflow sensors with ultra low-pressure drop.
BACKGROUND OF THE INVENTION
Flow rate control mechanisms are used in a variety of flow systems
as a means for controlling the amount of fluid, gaseous or liquid,
traveling through the system. The flow control mechanisms can be
utilized to regulate flow rates in systems such as ventilators and
respirators where, for example, it may be desirable to maintain a
sufficient flow of breathable air or provide sufficient
anesthetizing gas to a patient in preparation for surgery.
MEMS based flow sensors can be utilized for measuring such flow
rates in a variety of commercial, industrial and medical
applications. In medical applications, for example, it is often
required to accurately measure the flow rates of fluids introduced
intravenously to medical patients and thereby control the flow rate
of such fluids. In such applications, flow control is an inherent
aspect of proper operation, which can be achieved in part by
utilizing the flow sensors to measure the flow rate of fluid within
the flow system.
Ventilators are medical devices for delivering a breathing gas to a
patient. Usually, ventilators employed in hospital critical care
units provide a supply of air enriched with oxygen for inspiration
by the patient, and may conventionally include controls for either
assisting or controlling breathing, exhaled volume indicators,
alarms systems, positive end expiratory pressure valves, pressure
indicators, gas concentration monitors, flow indicators, and heated
humidifiers for warming and humidifying the breathing gas.
Ventilators used in home care are often used to treat obstructive
sleep apnea and supply positive air pressure to assist breathing.
Manufacturers of medical ventilator equipment require an ultra low
pressure drop to insure efficient blower operations.
The majority of prior art airflow sensors utilized for medical
ventilators operate on a principle of a flow restrictor, traversing
the air stream and measuring the pressure at a number of locations
in the duct. The static pressure drives a sample of airflow through
a bypass channel where the flow rate is measured.
An alternate technology uses a pitot tube having a probe with an
open tip, which is inserted, into the flow field in order to
measure a static pressure. The static pressure is an increasing,
continuous function of the airflow rate within the tube. The pitot
tube extends completely through the main channel of the sensor
therefore presents a barrier to the oncoming flow. The problem
associated with these sensors is that the sensor itself is
responsible for a certain amount of turbulence in the flow channel.
Sensor-generated turbulence causes an increase in pressure drop
across the sensor as well as noise in the duct system.
Based on the foregoing it is believed that a need exists for an
improved airflow sensor that reduces pressure drop and that is
adapted to reduce obstruction to the flow. It is believed that the
improved flow sensor disclosed herein can address these and other
continuing needs.
BRIEF SUMMARY
The following summary is provided to facilitate an understanding of
some of the innovative features unique to the embodiments disclosed
and is not intended to be a full description. A full appreciation
of the various aspects of the embodiments can be gained by taking
the entire specification, claims, drawings, and abstract as a
whole.
It is, therefore, one aspect of the present invention to provide
for improved sensor methods and systems.
It is another aspect of the present invention to provide for
improved airflow sensor with low-pressure drop.
The aforementioned aspects and other objectives and advantages can
now be achieved as described herein. An airflow sensor apparatus
for measuring flow rate includes a pitot tube with a bypass channel
wherein the pitot tube extends halfway into a flow channel in order
to reduce a pressure drop. One or more upstream taps can be spaced
along the pitot tube facing into a direction of a flow stream which
directs the flow to the bypass channel. At least one or more
downstream taps can be located to face perpendicular to the
direction of the flow, such that the fluid after passing over a
flow sensor passes through the downstream tap(s). The upstream tap
senses stagnation pressure and the down stream tap senses static
pressure which is exerted in all directions in the flow channel in
order to determine a velocity pressure based on a difference
between pressures.
The upstream taps and the downstream taps average the pressure in
the bypass channel in order to provide a more accurate reading of
the flow in the flow channel. The pitot tube extends halfway into
the flow channel hence the obstruction to the flow channel and the
pressure drop can be reduced. In addition, this technique of
sensing velocity pressure eliminates the need to add pressure drop
to the system to measure flow such as with a flow restrictor or
orifice. The velocity pressure can be sensed electronically
utilizing the flow sensor or ultra low-pressure sensor. The
orientation of the upstream taps and the downstream taps in this
configuration produces the difference between stagnation pressure
and drag pressure, which can be correlated to the flow of the
fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying figures, in which like reference numerals refer to
identical or functionally-similar elements throughout the separate
views and which are incorporated in and form a part of the
specification, further illustrate the embodiments and, together
with the detailed description, serve to explain the embodiments
disclosed herein.
FIG. 1 illustrates a perspective view of an airflow sensor
apparatus, which can be adapted for use in implementing a preferred
embodiment;
FIG. 2 illustrates a cut-away sectional view of the airflow sensor
apparatus, in accordance with a preferred embodiment;
FIG. 3 illustrates a cross sectional view of an airflow sensor, in
accordance with a preferred embodiment; and
FIG. 4 illustrates a cross sectional view of an airflow sensor, in
accordance with a preferred embodiment; and
FIG. 5 illustrates a cut-away sectional view of an exemplary
airflow sensor apparatus, in accordance with a preferred
embodiment.
DETAILED DESCRIPTION
The particular values and configurations discussed in these
non-limiting examples can be varied and are cited merely to
illustrate at least one embodiment and are not intended to limit
the scope thereof.
Referring to FIG. 1 a perspective view of an airflow sensor
apparatus 100 is illustrated, in accordance with a preferred
embodiment. The airflow sensor apparatus 100 includes a housing 110
defining a flow channel 125 into which an entering fluid may flow
from a flow system. Note that as utilized herein the term "fluid"
can refer to a liquid or a gas. The flow channel 125 can be defined
by a flow channel wall 115. The flow channel 125 preferably has a
cross-sectional shape and size compatible with that of existing
flow systems, such as to fit conical connector as used in
ventilators and respirators. A pitot tube 130 extends halfway into
the flow channel 125. The pitot tube 130 includes one or more
upstream taps 141, 142, 143, 144, 146, 147, 148 and 149 shaped with
a circular cross-sectional shape that can be oriented upstream in
flow channel 125 to face into the direction of fluid (e.g., gas)
flow.
The taps 141, 142, 143, 144, 146, 147, 148 and 149 can be
implemented as upstream taps that face into the flow of fluid. As
utilized herein, the term "tap" can refer to a small opening that
permits flow of a liquid or gas. The pitot tube 130 additionally
includes one or more downstream taps (not shown) that face
perpendicular to the fluid flow or opposite the direction of the
flow. The downstream taps function as exhaust taps for the bypass
channel.
Referring to FIG. 2 a cut-away sectional view of the airflow sensor
apparatus 100 shown in FIG. 1 is illustrated, in accordance with a
preferred embodiment. The fluid can pass through the flow channel
125 in the direction as indicated by arrow 126 via an inlet end
155, with the fluid exiting the flow channel 125 at an outlet end
165. A flow sensor die 190 is disposed in a bypass-sensing channel
170 parallel to the flow channel 125. The sensor die 190 and the
flow channel 125 are located adjacent which is protected by a cover
116, which in turn is situated within a housing 110. A cover 116,
disposed against the rear side of the substrate 117 opposite the
sensor die 190 protects the sensor die 190 from environmental
effects.
The pitot tube 130 is disposed in the flow channel 125 and aligned
perpendicular to the flow channel 125. The pitot tube 130 has a
leading edge 131 and is preferably sharpened as seen in FIG. 1 such
that the fluid smoothly enters the tube 130 thus minimizing or
substantially reducing turbulence and droplet shear. The pitot tube
130 extends halfway into the flow channel which reduces pressure
drop and obstruction to flow. The upstream taps 141,142,143 and 144
in the pitot tube 130 leads to the upstream low resistance flow
channel or port 150, which can direct bypass flow to the sense die
190. After passing over the sense die 190, the bypass flow of fluid
continues in a downstream low resistance flow path or port 160, and
exhausts through downstream taps 146, 147, 148 and 149, which are
oriented opposite the direction of flow.
The fluid flows through the flow channel 125 in the direction
indicated by arrow 126, a portion of the fluid flows through the
upstream taps 141, 142, 143 and 144 in the pitot tube 130 to the
bypass channel 170 so that the flow sensor die 190 can measure the
flow rate of the fluid in the flow channel 125 indirectly without
being exposed to the damage or fluctuating conditions existing in
typical flow channels.
Referring to FIG. 3, a cut-away sectional view of the airflow
sensor apparatus 200 is illustrated wherein downstream taps 146,
147, 148 and 149 are shown oriented away from the flow
direction.
Referring to FIG. 4 a cross sectional view of an airflow sensor 300
is illustrated, in accordance with a preferred embodiment. The
sensor 300 includes an upstream sensing element 191, a downstream
sensing element 192 and a central heating element 193. The heating
element 193 and sensing elements 191 and 192 comprises of resistive
thin films (not shown), which comprise an electrical bridge whose
output is analogous to the differential pressure applied to the
sensor apparatus 100 illustrated in FIGS. 1-2. The sensing elements
191 and 192 can be implemented as, for example, a MEMS type airflow
sensor. It can be appreciated, of course, that the sensing elements
191 and 192, may be configured in the context of other types of
sensor designs, not merely MEMS-type configurations. The fluid will
flow across the upstream sensing element 191, the downstream
sensing element 192 and the heating element 193. Under no flow
conditions, the upstream sensing element 191 and the downstream
sensing element 192 would both read the same temperature due to the
heating element 193, i.e., both sensors would have the same
measured resistance values.
As the fluid enters the upstream port 150 of the pitot tube 130,
the upstream sensing element 191 senses the average sensor impact
pressure of flowing fluid to establish a high pressure value
resulting in a reduction of temperature. The downstream sensing
element 192 senses low pressure, which forms in downstream port 160
of the bypass channel 170 resulting in an increase of temperature.
The change in temperatures produces a corresponding change in the
resistance values of the sensor 300. The sensor 300 transforms the
respective high and low fluid pressures into an electrical signal
whose character is a function of the differential pressure (DP),
that is the difference between the sensed high and low fluid
pressures. Upstream taps 141, 142, 143 and 144 and the downstream
taps 146, 147, 148 and 149, such as those illustrated in FIGS. 2-3,
can average the pressure in the bypass channel 170 in order to
provide a more accurate reading of the flow in the flow channel
125.
Note that as utilized herein the acronym "MEMS" refers generally to
term "Micro-electro-mechanical Systems". MEMS devices refer to
mechanical components on the micrometer size and include 3D
lithographic features of various geometries. They are typically
manufactured using planar processing similar to semiconductor
processes such as surface micromachining and/or bulk
micromachining. These devices generally range in size from a
micrometer (a millionth of a meter) to a millimeter (thousandth of
a meter). At these size scales, a human's intuitive sense of
physics do not always hold true. Due to MEMS' large surface area to
volume ratio, surface effects such as electrostatics and wetting
dominate volume effects such as inertia or thermal mass.
MEMS devices can be fabricated using modified silicon fabrication
technology (used to make electronics), molding and plating, wet
etching (KOH, TMAH) and dry etching (RIE and DRIE), electro
discharge machining (EDM), and other technologies capable of
manufacturing very small devices. MEMS sometimes go by the names
micromechanics, micro machines, or micro system technology (MST).
While the inserted position of the sensor shown in FIGS. 1-3 are
preferred for the illustrated design of pitot tube 130, other
configurations are possible and may even be favored for pitot tubes
of different design and configuration.
Referring to FIG. 5 cut-away sectional view of the airflow sensor
apparatus 400 is illustrated, in accordance with an alternative,
but preferred embodiment. Note that in FIGS. 1-4, identical or
similar parts or elements are generally indicated by identical
reference numerals. The design of apparatus 400 thus includes one
or more upstream taps 141, 142, 143 and 144, which face into the
flow direction 126 as shown in FIG. 2 that is to be measured. The
upstream taps 141, 142, 143 and 144 can direct the flow of fluid to
the sensor die 190 (e.g., MEMS sensor). The sensor die 190 and the
bypass channel 170 are located adjacent a housing 110.
The pitot tube 130 upstream taps 141, 142, 143 and 144 creates a
pressure drop across the upstream port 150 and down stream port 160
of the bypass channel 170 which facilitates fluid flow into the
bypass channel 170. This pressure drop, or pressure differential,
is dependent on pitot tube 130 geometry and increases with flow
rate. Furthermore, the fluid in the flow channel 125 will have an
increasingly turbulent flow as the flow rate of the fluid
increases, i.e., an increasing non-uniform pressure and velocity
across a given plane orthogonal to the direction of flow. In
response, by reducing the pitot tube 130 to half of the flow
channel 125 reduces pressure drop, straightens and luminaries the
flow in the flow channel 125, thereby reducing turbulence. The
pitot tube 130 reduces turbulence by forcing the fluid to flow
through other half of the flow channel 125. The pitot tube 130 can
incorporate an up and/or downstream flow straightener(s) (131)
protruding into the flow to enhance flow stability.
It will be appreciated that variations of the above-disclosed and
other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or
applications. Also that various presently unforeseen or
unanticipated alternatives, modifications, variations or
improvements therein may be subsequently made by those skilled in
the art which are also intended to be encompassed by the following
claims.
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